Axial flow gas turbine engines include a compressor, a combustor, and a turbine spaced sequentially along a longitudinal axis. An annular flow path extends axially through the compressor, combustor and turbine. The compressor includes an array of rotating blades that engage incoming working fluid to compress the working fluid. A portion of the compressed working fluid enters the combustor where it is mixed with fuel and ignited. The products of combustion or hot gases then flow through the turbine. The turbine includes alternating arrays of vanes and rotating blades. In the turbine, energy is transferred from the flowing hot gases to the turbine blades. A portion of this energy is then transferred back to the compressor section via a rotor shaft.
To optimize the efficiency of the interaction between the turbine blades and the hot gases flowing through the turbine, the hot gases are confined to an annular space defined by inner and outer turbine shrouds. The inner turbine shroud is typically a plurality of platforms integral to the blades. The platforms mate with adjacent platforms to form an inner flow surface for the hot gases. The outer shroud is typically a ting-like assembly disposed radially outward of, but in close radial proximity to, the outer tips of the rotating blades. The assembly includes a plurality of arcuate segments spaced circumferentially to provide an outer flow surface for the hot gases.
The segments include a substrate and a coating layer defining a flow surface for the segment. The substrate includes the means to retain the segment to the turbine, typically a plurality of hooks or a rail disposed along the leading edge and trailing edge of the segment. The coating layer may be a thermal barrier coating and/or an abradable coating. The thermal barrier coating provides insulation for the segment against the hot gases flowing through the turbine. The abradable coating provides material for the tips of the blades to engage with during operation. The tips may be coated with a abrasive coating that cuts into the abradable coating to minimize the amount of hot gases which leak around the blades. The combination of abrasive tips and abradable coating prevent damage to the blades and substrate during contact. An example of such a segment is disclosed in U.S. Pat. No. 4,650,395, entitled "Coolable Seal Segment For a Rotary Machine" and issued to Weidner.
A common problem with segments of the type described above is spalling of the coating layer. Spalling refers to the coating layer detaching from the substrate. Spalling occurs as a result of thermal stresses within the segment. A thermal gradient exists across the segment due to the hot gases present on the coating side and a supply of cooling fluid flowing over the radially outer surface of the substrate. The differing rates of thermal expansion between the metal of the substrate and the material of the coating layer adds to the stresses present within the segment.
Spalling exposes the bare metal of the substrate to the hot gases and/or abrasive contact with the tip of the blade. Besides the potential degradation of the segment, spalling may also increase the size of the gap between the tips of the blades and the segments. Increases in the gap provides an opening for hot gases to flow around the blades and reduces the efficiency of the turbine.
The above art notwithstanding, scientists and engineers under the direction of Applicants' Assignee are working to develop turbine components, such as turbine shroud segments, capable of operating within extreme temperature environments for extensive periods of time with minimal degradation of the component.